Heat - discs are preferred to electric heating pads which can catch fire or just get too hot. (doctorpup.com)
The BodyMed Electric Heating Pad is a moist heat therapy heating pad that doesn't require you to add the water yourself: it uses the humidity of the air to create moist heat! (geekwrapped.com)
Electric heating pad cautions actually read: Never use in a situation where you could fall asleep. (sew4home.com)